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CVE-2025-39821

Estado: ModificadaAlta (7.8)—

In the Linux kernel, the following vulnerability has been resolved:

perf: Avoid undefined behavior from stopping/starting inactive events

Calling pmu->start()/stop() on perf events in PERF_EVENT_STATE_OFF can leave event->hw.idx at -1. When PMU drivers later attempt to use this negative index as a shift exponent in bitwise operations, it leads to UBSAN shift-out-of-bounds reports.

The issue is a logical flaw in how event groups handle throttling when some members are intentionally disabled. Based on the analysis and the reproducer provided by Mark Rutland (this issue on both arm64 and x86-64).

The scenario unfolds as follows:

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The throttling mechanism attempts to start/stop events that are not actively scheduled on the hardware.

Move the state check into perf_event_throttle()/perf_event_unthrottle() so that inactive events are skipped entirely. This ensures only active events with a valid hw.idx are processed, preventing undefined behavior and silencing UBSAN warnings. The corrected check ensures true before proceeding with PMU operations.

The problem can be reproduced with the syzkaller reproducer:

Detalles técnicos trazas, registros y código del informe original
 1. A group leader event is configured with a very aggressive sampling
    period (e.g., sample_period = 1). This causes frequent interrupts and
    triggers the throttling mechanism.
 2. A child event in the same group is created in a disabled state
    (.disabled = 1). This event remains in PERF_EVENT_STATE_OFF.
    Since it hasn't been scheduled onto the PMU, its event->hw.idx remains
    initialized at -1.
 3. When throttling occurs, perf_event_throttle_group() and later
    perf_event_unthrottle_group() iterate through all siblings, including
    the disabled child event.
 4. perf_event_throttle()/unthrottle() are called on this inactive child
    event, which then call event->pmu->start()/stop().
 5. The PMU driver receives the event with hw.idx == -1 and attempts to
    use it as a shift exponent. e.g., in macros like PMCNTENSET(idx),
    leading to the UBSAN report.

CVSS

Probabilidad de explotación (EPSS)

EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).

🎯 Técnicas ATT&CK

Cómo se explota esta vulnerabilidad y qué consigue el atacante, en el lenguaje de MITRE ATT&CK.

Vulnerabilidad local (AV:L/PR:L) en kernel Linux que permite escalada de privilegios mediante operaciones de bitwise con índices inválidos. Permite ejecución arbitraria o DoS al manipular eventos de PMU.

Inferido por nuestro agente de análisis a partir de la descripción oficial, el vector CVSS y la CWE, y comprobado por un supervisor. Puede contener errores.

🛡️ Mitigaciones ATT&CK que cubren estas técnicas

Execution PreventionThreat Intelligence ProgramRestrict Web-Based ContentPrivileged Account ManagementLimit Software InstallationFilter Network Traffic

Tecnologías afectadas (1)

Linux — Linux Kernel

CWE

Referencias

JSON original (NVD)

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